Above-ground habitat construction by plant shoots

By intercepting solar radiation, plant shoot systems determine both the quantity and spectral quality of light that reaches their less lofty neighbors and their own lower leaves (H. Smith 1982; Valladares and Niinemets 2008; and references therein). These effects on light conditions are determined by plant variation at several levels: species-specific aspects of shoot and leaf form, individual height, architecture, and age, as well as the size, angle, and anatomy of leaves (Falster and Westoby 2003; Bartemucci et al. 2006). The interception of solar energy also creates altered temperature and humidity conditions within and beneath plant shoot systems (Fitter and Hay 2002). The light attenuation and associated microclimatic changes below plant canopies modulate the germination and early growth environment for their own, and their neighbors', offspring (Bazzaz 1996). As a result of these effects, plant community composition and dynamics are a cause as well as a consequence of variation in light conditions (Val- ladares and Niinemets 2008). Living plant shoots also shape moisture conditions for themselves and their neighbors, both through evapotranspiration from leaf surfaces (as this process elevates ambient humidity) and by redistributing rainfall as it falls through the complex layers of the shoot system (Fitter and Hay 2002).

Individual plants further alter the microclimate of the nearby soil surface by seasonally depositing vegetative litter. This litter consists primarily of dead leaves but includes as well dried reproductive parts and broken bits of twig and bark (March and Watson 2007; Figure 5.4). This layer of plant detritus physically reduces the amount of direct light that reaches the soil surface zone and increases its humidity and temperature, influencing plant germination and seedling development (Stinchcombe and Schmitt 2006 and references therein). Even small amounts of plant litter can increase soil moisture and thus indirectly improve plant growth conditions (Foster and Gross 1998). By reducing light levels, the deposition of leaf litter can influence community composition as well, either directly or via the suppression of otherwise dominant competitors (Facelli 1994; Foster and Gross 1998).

Vegetative litter deposited by plants, such as dead leaves and broken twigs, strongly affects light, temperature, humidity, and nutrient cycling at and below the soil surface

Figure 5.4 Vegetative litter deposited by plants, such as dead leaves and broken twigs, strongly affects light, temperature, humidity, and nutrient cycling at and below the soil surface. The timing, amount, and tissue composition of litter deposits have consequences for seed germination, nutrient cycling, and soil biota that scale up to the ecosystem level. photo of autumn leaf litter in a mixed deciduous forest (four Marks, hampshire, UK), courtesy of Chris Rose, http://fourmarksbirding.blogspot.com; for the color image, see plate 16.

Beyond these straightforward physical effects, plant litter plays a surprisingly important role in shaping nutrient conditions below the ground. The decomposition of these discarded vegetative parts (whether or not hastened by earthworm burial as described above) creates nutrient inputs near the plant, including pulses of released carbon and nitrogen, that alter the structure, chemistry, and biota of the soil (Madritch and Hunter 2005). In conjunction with the physical impacts of increased temperature and soil moisture, these chemical inputs speed up rates of mineralization, promoting arthropod and bacterial populations and ultimately cycling back into vegetation (Hodge 2004; March and Watson 2007). Accordingly, the deposition of litter is a critical determinant of nutrient availability and cycling in natural communities, with profound effects on ecosystem productivity and diversity (Wolkovich 2010).

Since tissue decomposition rates are influenced by tannin concentration and other specific aspects of plant secondary chemistry, the chemical effects of vegetative litter vary depending on plant species, population, and genotype (Madritch and Hunter 2005). Indeed, one indirect consequence of reduced biodiversity in plant communities (and of altered atmospheric chemistry, soil conditions, and other factors that influence plant tissue composition) may be changes in mineral cycles resulting from altered leaf litter chemistry (discussed in Madritch and Hunter 2005). Conversely, the introduction of new plant species to terrestrial communities can result in either negative or facilitative effects via their leaf litter, a generally ignored aspect of biological invasion. For instance, the invasion of a shrub-dominated, semiarid California coastal area by a nonnative annual grass increased soil moisture in the system, as formerly bare soil was covered by grass plants and their copious leaf litter (Wolkovich 2010). In this moisture-limited system, leaf litter deposits by an introduced plant enhanced both the growth of the native flora and that of the associated arthropod community of shrub herbivores and their spider predators (Wolkovich 2010).

In some cases, leaf litter can influence community composition by means of specific allelo- pathic effects. Chemicals that leach from the fallen leaves of the sub-Arctic heath plant Empetrum hermaphroditum have been experimentally shown to suppress the growth of pine seedlings, for instance (Gallet et al. 1999; additional references in Ridenour and Callaway 2001).

One well-studied example of the habitatconstructing effects of leaf litter is the Australian mistletoe plant Amyema miquelii, whose distinctive life history causes marked changes in ecosystem nutrient dynamics. This endemic hemiparasite and its congener Amyema pendula are commonly found growing on the branches of various dominant species of Eucalyptus tree across Australia, a region with generally nutrient-poor soils (Figure 5.5). Although individual Amyema miquelii plants are small relative to the host tree's canopy, their extremely rapid leaf turnover results in a very high production of leaf litter (March and Watson 2007). Indeed, much of the live leaf biomass of the mistletoe plants is deposited as litter every year, since their leaves live only 1.5 years on average (compared with over 4 years for Eucalyptus leaves; March and Watson 2007). As a result of this rapid turnover, the leaf litter below host trees consists disproportionately of the leaves of their mistletoe parasites, and the total amount of leaf litter beneath trees that host mistletoes is more than double that of mistletoe-free trees (March and Watson 2007).

Like other parasitic and hemiparasitic plants, mistletoes accumulate and concentrate the host's resources in their own tissues (Press 1998 and references therein). Consequently, mistletoe leaf litter contains higher concentrations of nitrogen, phosphorous, and other minerals than does litter from Eucalyptus trees (the amount and nutrient content of which is not significantly affected by mistletoe presence; March and Watson 2010). The greater quality and quantity of mistletoe leaf litter combine to create extremely nutrient-rich patches directly below host trees. In many such patches, twice the nitrogen, four times the phosphorous, and ten times the potassium are returned to the soil than is returned via litter from Eucalyptus trees alone; the mineral returns can be even higher depending on the size and number of mistletoe plants present (March and Watson 2010). These mistletoe litter inputs substantially alter the dynamics of nutrient cycling in this system, as minerals previously held in the biomass of long-lived woody host plants are rapidly

Australian mistletoe plants

Figure 5.5 Australian mistletoe plants (Amyema miquelii and Amyema pendula) are hemiparasites of the native Eucalyptus trees. The mistletoes redistribute soil nutrients both spatially and temporally, acting as vegetative "Robin hoods" by accumulating resources from their host trees and depositing them via leaf litter. Although individual mistletoe plants are small relative to the host tree's canopy, their extremely rapid leaf turnover results in very high leaf litter deposition rates. Photo of Amyema pendula on Eucalyptus sp. (Kimberly region, Western Australia), courtesy of Clare Morton.

returned to the soil. Moreover, the seasonal timing of mistletoe litterfall occurs primarily when their Eucalyptus host trees produce the least litter, so mistletoes also extend the period of nutrient inputs to soil (March and Watson 2007).

Hemiparasitic Australian mistletoes thus can be seen as vegetative "Robin Hoods" that raise soil productivity by spatially and temporally redistributing nutrients, creating locally rich soils that promote the growth of smaller organisms (including tree seedlings) and extending the seasonal availability of key minerals (Press 1998; March and Watson 2010). Root hemiparasites of nutrient- poor grasslands and sub-Arctic communities apparently play a similar role in increasing soil productivity through enhanced nutrient cycling (references in March and Watson 2007). It may be that this habitat-constructing role is a key to the ecology of plant-parasite interactions; the 3,000 to 4,000 known species of parasitic and hemipa- rasitic flowering plants are indeed most common in nutrient-limited ecosystems (Press 1998). More broadly, the spatial and temporal redistribution of mineral nutrients and organic matter from long- lived plants via their depositions of vegetative litter is one way that plants in general influence their environments.

 
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